Page 287 - Fundamentals of Enhanced Oil and Gas Recovery

Enhanced Oil Recovery (EOR) in Shale Oil Reservoirs 275

2.5 mD. Considering these points can help to have a better evaluation and comparison
between huff-n-puff CO 2 injection and continuous carbon dioxide flooding.
According to their simulation results, enriching carbon dioxide improve the efficiency
of the injection process. Also, the results revealed that the sweep efficiency of water
flooding is much lower than CO 2 injection scenario. Their results confirmed those
ones reported by Sheng and Chen [11], Joslin et al. [16], and Dong and Hoffman [17]
for the Bakken Formation in the Sanish Field, North Dakota.

9.3.1.2 Huff-n-Puff Gas Injection
Sheng and Chen [11] proposed the use of CO 2 huff-n-puff scenario instead of contin-
uous mode because of in a case of continuous injection the pressure in a near injec-
tion well region increases dramatically and near production well area the pressure
decreases significantly due to the tightness of the rock. These points lead them to sug-
gest the huff-n-puff scenario in such a reservoir [18] and they verified this recommen-
dation with the experiments [19]. Furthermore, various experimental works have
been done to evaluate this proposal [20 24]. In such a scheme there are different
parameters should be optimized, such as number of cycles, injection time, soaking
period, production period, and well configurations. In most of the researches, which
have been performed in both lab scale and pilot/field scale, the shorter soaking time
resulted in higher oil recovery factor; the best case scenario was zero soaking period
in such a method [18,25 27]. It is worth to highlight that this observation works for
a case of huff-n-puff with several cycles; it is obvious for a single cycle more soaking
time yield more oil recovery factor [28,29]. In laboratory experiments the effect of
soaking period when we dealing with gas condensate samples is ignorable [30,31].
The performance of carbon dioxide flooding coud be improved by enriching the
composition of the injected gas; carbon dioxide is a promising EOR agent, especially
in shale oil reservoirs [32]. Carbon dioxide injection scenario is employed broadly
throughout world to improve oil recovery factor from different type of oil resources
including naturally fractured reservoirs, deep and shallow conventional reservoirs, and
tight reservoirs; however, in all these projects two drawbacks associated with carbon
dioxide: (1) Availablity in huge volume, (2) Corrosion of the surface and downhole
facilities in case of using conventional materials.

9.3.1.3 Advantages and Drawbacks of Gas Injection
Gas flooding is much more commonly used than huff-n-puff gas injection in conven-
tional reservoirs. However, in shale or tight reservoirs, because of ultralow permeabil-
ity and thus a significant pressure drop in the matrix, it is very difficult for the gas to
drive oil from an injector to a producer. If a shale or tight reservoir has natural frac-
ture networks or the hydraulic fractures connect an injector and a producer, gas will
easily break through, resulting in a very low sweep efficiency [1,11]. There is no such